While the rates of tobacco smoking are on the decline, the frequency of marijuana use is rising and especially high among individuals at risk for HIV-1. In this setting it is important to establish whether marijuana smoking is associated with adverse health effects. Cannabinoids are known to modulate immune function when tested in vitro and in animal models. However, the extent to which these effects occur in marijuana smokers and their functional consequences are not known. Epidemiologic data variably identify marijuana use as a risk factor for HIV and pneumonia and our research has shown that alveolar macrophages (AM) recovered from the lungs of marijuana smokers are functionally impaired. However, studies examining the effects of marijuana smoking on systemic immune function have not demonstrated clear deficiencies. This discrepancy between pulmonary and systemic findings leads us to hypothesize that a localized response to marijuana smoking occurs in the lung, results from exposure to high local concentrations of THC, and polarizes AM function along a pathway that cannot adequately protect against HIV infection or pneumonia. This grant proposes three specific aims: Aim 1 will determine whether THC delivered by marijuana smoke exerts a local effect on AM function, in the absence of effects on blood monocytes, and identify the specific host defense pathways that are disrupted. Broncho- alveolar lavage and peripheral blood sampling will be carried out with a cohort of marijuana smokers and controls (both non-smokers and tobacco smokers) to obtain purified AM and monocytes. These cells will be assessed at baseline and when challenged with a toll-like receptor ligand to determine the impact of marijuana use on their expression of receptors, signaling pathways and effector mechanisms involved in monocyte and macrophage host defense. The direct role of THC and cannabinoid receptors in mediating these effects will be investigated in vitro. Aim 2 will investigate the effects of chronic marijuana use on the susceptibility of AM to HIV infection. AM obtained from habitual marijuana smokers will be compared to those from controls for their susceptibility to HIV infection and tested for their capacity to transmit HIV to autologous T cells Effector pathways that are altered in marijuana smokers will be assessed for their role in viral susceptibility, proliferation and transmission. Aim 3 will examine the relationship between the route of THC exposure (smoking vs. systemic injection), local THC concentrations, and susceptibility to bacterial pneumonia in a mouse model of chronic marijuana smoking. Mice will be exposed to marijuana smoke or to intra-peritoneal THC and dosing adjusted so to achieve serum THC levels similar to those occurring in marijuana smokers. Treatment groups will be compared for their responses to an intra-tracheal challenge with S. aureus under both naive and post- viral conditions. The role of cannabinoid receptors will be evaluated using receptor knockout strains. By the conclusion of these studies we will have a much clearer understanding of marijuana-related toxicity and its potential for health-related consequences in marijuana smokers and those at risk for HIV.